1. Field of the Invention
The present invention relates to a wafer holder and a method for manufacturing the same.
2. Description of the Related Art
In wafer holders in which a ceramic electrostatic chuck (electrostatic chuck heater, susceptor) is bonded to a metal cooling plate, a resin adhesive tape or an adhesive material is typically used for bonding the electrostatic chuck to the cooling plate (for example, PTL 1). The operation temperature of such a wafer holder is limited on the basis of the heat resistance of the resin (in general, 100° C. or lower, and even in the case of high-temperature grade, 150° C. or lower at the best). It is difficult to use a heat-resistant resin as the wafer holder because the modulus of elasticity of the resin is high and a difference in thermal expansion between the ceramics and the cooling plate cannot be absorbed, so that deformation or cracking occurs. In the case of joining with a brazing metal or the like, it is possible to use at high temperatures insofar as joining can be conducted. However, if the thermal expansion of the ceramics and the thermal expansion of the cooling plate (metal) are not equalized with each other, deformation or cracking occurs in the time of joining.
On the other hand, PTL 2 discloses an apparatus in which a heat-resistant layer, an adhesive layer, and a heat-insulating layer are disposed between a ceramic substrate and a base plate including a water channel. The heat-resistant layer is made of, for example, an insulating resin, e.g., polyimide, or an inorganic adhesive containing an inorganic material, e.g., low-melting glass, alumina, or silica, as a base. The adhesive layer and the heat-insulating layer are made of insulating resins, e.g., silicone. In order to produce such an apparatus, initially, the heat-insulating layer is formed on the base plate. Specifically, an insulating resin, e.g., silicone, including a filler is laminated on the base plate, and the resin is cured by heating to a curing temperature or higher while being pressed, as necessary. Subsequently, the heat-resistant layer is formed on one surface of the ceramic substrate. When forming the heat-resistant layer, a material on a film may be laminated and cured, or a liquid or paste material may be applied and cured. Then, the adhesive layer is formed on the heat-insulating layer. Before the adhesive layer is cured, the ceramic substrate is mounted on the adhesive layer so that the heat-resistant layer side of the ceramic substrate comes into contact with the adhesive layer. Thereafter, the adhesive layer is cured by being heated to the curing temperature or higher. The operation temperature of such an apparatus can be increased than ever. Also, the adhesive layer having the flexibility is disposed and, therefore, even in the case where a stress is generated at the time of heating, the resulting stress can be relaxed by the adhesive layer.